The present invention relates to nuclear reactors and, more particularly, to tools for removing and installing control rod drives for commercial power nuclear reactors.
A boiling-water nuclear reactor employs a plurality of fuel rods containing a nuclear fuel within a reactor vessel. The reactor vessel is filled with water to a level at least sufficient to cover the fuel rods. Fission in the fuel rods releases heat that boils the water surrounding them. This steam is used, either directly, or through an intermediate heat exchanger, to perform a useful function such as, for example, driving an electric turbine-generator.
The intensity of the nuclear reaction in a nuclear reactor is controlled, in part, by moving control rods between fuel rods. The control rods absorb neutrons, thereby controlling the intensity of the nuclear reaction, and the rate at which steam is produced.
The control rods are controlled by control rod drives inserted through the bottom of the reactor vessel. Control rod drives occasionally require maintenance or replacement. This has presented a problem because of the structure of the control rod drives and the working environment in which they must be handled.
A typical control rod drive is about 16 feet long and weighs about 450 pounds. It is thus an awkward device that requires substantial mechanical handling assistance to install and remove. In addition, the sub-pile room below the reactor vessel typically has a headroom between the floor and the bottom of the reactor vessel of about 18 feet. This leaves little maneuvering room for lowering the control rod drive, rotating it into a horizontal position, and moving it out of the sub-pile room. Also, numerous fragile instrumentation cables hang down from the bottom of the reactor vessel. Such instrumentation cables can be damaged by contact with a control rod drive. If an instrumentation cable is damaged, the rules governing operation of a nuclear reactor require that work must stop until the damaged instrumentation cable is repaired.
A further problem arises because the sub-pile room below a nuclear reactor is a high-radiation area. It is thus desirable to limit the amount of time that workers spend in that area.
The following publications relate to devices which are used to lower and rotate a control rod drive in the sub-pile room. All of these publications are in Japanese, and full translations are not available. A translation of claim 1 is available and is provided for the use of the Patent and Trademark Office:
Japanese Patent Publication No. 60-48715 PA0 Japanese Patent Publication No. SHO-60-49277 PA0 Japanese Patent Publication No. SHO-61-31839 PA0 Japanese Patent Publication No. 58-32359 PA0 Japanese Patent Publication No. SHO-61-36636 PA0 Japanese Patent Publication No. 61-42838 PA0 Japanese Patent Publication No. SHO-61-42839 PA0 Japanese Patent Publication No. SHO-61-36635 PA0 Japanese Patent Publication No. SHO-61-33158 PA0 Japanese Patent Publication No. SHO-61-25116 PA0 Japanese Patent Publication No. SHO-61-13198 PA0 Japanese Patent Publication No. SHO-57-39398 PA0 Japanese Patent Publication No. 57-49833 PA0 Japanese Patent Publication No. SHO-58-27880 PA0 Japanese Patent Publication No. 59-31034 PA0 Japanese Patent Publication No. SHO-60-35035 PA0 Japanese Patent Publication No. SHO-60-35036 PA0 Japanese Patent Publication No. 60-37439 PA0 Japanese Patent Publication No. SHO-60-46676
The length of the above list is regretted. However, the spirit of full disclosure requires the inclusion of each reference of which the applicants are aware.
Also, as the seal between a control rod drive and the reactor vessel is broken during removal, a small amount of residual water spills from in the reactor vessel. Usually, the spilling water, which is contaminated with radioactivity, falls upon a worker in the process of removing the control rod drive. Although workers wear protective clothing and breathing apparatus in this area, it is considered undesirable to permit residual water to fall upon them. Japanese Utility Model Application Publication No. 57-49834, and Japanese Patent Publication Nos. SHO-58-15759 and SHO-53-18676 disclose water drain apparatus for use with control rod drives.
Bolts securing a control rod drive are highly torqued during installation. Due to the cramped conditions in the sub-pile room, it is difficult to maneuver suitable tools into place to detorque these bolts to enable their removal. Japanese Patent Publication Nos. SHO-61-22274 and SHO-61-22275 disclose tools designed to remove such bolts.